RFC 7587：Opus语音与音频编解码器的RTP封装规范

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RFC-7587《实时传输协议（RTP）封装Opus语音和音频编码格式》是由Internet Engineering Task Force (IETF) 的J. Spittka和K. Vos以及Mozilla的JM. Valin共同编写的标准化文档，发布于2015年6月。该RFC的主要目标是定义Opus编码的语音和音频数据在RTP协议中的有效封装方式，以便实现Opus在RTP环境中的无缝集成，并提供其在RTP传输中的适用性声明。 RFC-7587的重要性在于它明确了如何在实时通信环境中处理Opus编码，这是一种高效且广泛用于语音和音频通信的编码技术。Opus能够根据网络条件自动调整比特率，适用于各种带宽，包括VoIP和视频会议场景。它将Opus编码后的数据封装到RTP包中，确保了跨不同平台和应用的兼容性。文档详细描述了RTP payload格式的设计，包括封装结构、标识符、采样率和时序等关键元素。这些规定旨在使终端设备和网络中间件能够正确识别并解码Opus数据，从而保证高质量的语音和音频通信体验。此外，RFC还对RTP payload格式进行了媒体类型注册，这是确保网络协议一致性的重要步骤。作为一项互联网标准跟踪文档，RFC-7587已经经过了公共审查，并得到了Internet Engineering Steering Group (IESG) 的批准。对于想要了解此文档当前状态、错误修正和反馈渠道的读者，可以通过访问http://www.rfc-editor.org/info/rfc7587 获取相关信息。版权方面，RFC-7587受版权保护，遵循特定的授权条款，确保所有使用和分发行为符合规定的许可协议。 RFC-7587为RTP协议与Opus编码的结合提供了明确的技术规范，促进了高质量的语音和音频在互联网上的实时传输，是IT行业在多媒体通信领域的重要参考资料。

RFC 7587 RTP Payload Format for Opus June 2015

3. Opus Codec

Opus encodes speech signals as well as general audio signals. Two

different modes can be chosen, a voice mode or an audio mode, to

allow the most efficient coding depending on the type of the input

signal, the sampling frequency of the input signal, and the intended

application.

The voice mode allows efficient encoding of voice signals at lower

bitrates while the audio mode is optimized for general audio signals

at medium and higher bitrates.

Opus is highly scalable in terms of audio bandwidth, bitrate, and

complexity. Further, Opus allows transmitting stereo signals with

in-band signaling in the bitstream.

3.1. Network Bandwidth

Opus supports bitrates from 6 kbit/s to 510 kbit/s. The bitrate can

be changed dynamically within that range. All other parameters being

equal, higher bitrates result in higher audio quality.

3.1.1. Recommended Bitrate

For a frame size of 20 ms, these are the bitrate "sweet spots" for

Opus in various configurations:

o 8-12 kbit/s for NB speech,

o 16-20 kbit/s for WB speech,

o 28-40 kbit/s for FB speech,

o 48-64 kbit/s for FB mono music, and

o 64-128 kbit/s for FB stereo music.

3.1.2. Variable versus Constant Bitrate

For the same average bitrate, variable bitrate (VBR) can achieve

higher audio quality than constant bitrate (CBR). For the majority

of voice transmission applications, VBR is the best choice. One

reason for choosing CBR is the potential information leak that

_might_ occur when encrypting the compressed stream. See [RFC6562]

for guidelines on when VBR is appropriate for encrypted audio

communications. In the case where an existing VBR stream needs to be

converted to CBR for security reasons, the Opus padding mechanism

Spittka, et al. Standards Track [Page 4]

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RFC 7587：Opus语音与音频编解码器的RTP封装规范

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